It is common practice to cryogenically treat natural gas to liquefy the same for transport and storage. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. Often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas which in turn requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to -240.degree. F. to -260.degree. F. where it possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas or the like in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane and mixtures thereof. In the art, the refrigerants are frequently arranged in a cascaded manner and each refrigerant is employed in a closed refrigeration cycle. When the condensed liquid is at an elevated pressure, further cooling is possible by flashing the liquefied natural gas to atmospheric pressure in one or more expansion stages. The flashing is generally accomplished via the use of expansion valves. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a storage or transport temperature suitable for liquefied gas storage at near-atmospheric pressure. In this expansion to near-atmospheric pressure, significant volumes of flash vapors are produced. The flash vapors from the expansion stages are generally collected and recycled for liquefaction or utilized as fuel gas for power generation.
In an open cycle cascaded refrigeration process, the cycle comprises the steps of flashing a pressurized LNG-bearing stream in discrete steps, warming the resulting flash vapor streams by employing such streams as refrigeration streams, recompressing a substantial portion of the resulting warmed flash vapor streams, cooling said compressed gas stream and returning the compressed cooled gas stream to the liquefaction process for liquefaction. As previously noted, the flashing of a pressurized LNG-bearing stream to near-atmospheric pressure is generally performed with expansion valves. From a thermodynamic perspective, such flashing is a highly irreversible process.